The present invention relates to an electromagnetic apparatus used for example in magnetic resonance imaging systems, more particularly to the structure for attaching magnetic substances referred to as passive shims to correct magnetic field inhomogeneity.
The prior art electromagnetic apparatus shown in FIG. 1 is known well. An exploded and enlarged oblique view of the part A of FIG. 1 is illustrated in FIG. 2. The prior art electromagnetic apparatus comprises a housing 1 containing coils for generating the magnetic field. The housing 1 has a cavity wherein a uniform magnetic field is to be created in a space 2. Non-uniform components of the field in this space 2 are corrected by passive shims 3 such as soft iron bars. The passive shims 3 are housed in non-magnetic tubes 4 which are attached to the inner surface of the housing 1.
To install the non-magnetic tubes 4 in the housing 1, a support unit 5 is attached to the housing 1. The support unit 5 has holes 5a into which the non-magnetic tubes 4 fit, and projecting seats 5b for attaching the support unit 5 to the housing 1 by screws 6. An end plate 7 closes the end openings of the non-magnetic tubes 4 to prevent the passive shims 3 from escaping from the non-magnetic tubes 4. The end plate 7 is fastened to the support unit 5 by screws 8.
The magnetic field in the cavity is adjusted according to measurements of the magnetic field intensity in the space 2. If the measurements indicate that further adjustment of the magnetic field homogeneity is necessary, the end plate 7 is removed from the support unit 5 and passive shims 3 are added or removed. After this, the end plate 7 is reattached to the support unit 5, and the distribution of the magnetic field is measured again. If necessary, the above procedure is repeated until satisfactory field homogeneity is obtained.
The adjustment is first performed using soft iron wire about 3 to 5 mm in diameter, then gradually finer soft iron wire is used. The finest soft iron wire has a diameter of 1 mm or less.
A problem with the prior art electromagnetic apparatus is that the passive shims must be inserted into small holes in the non-magnetic tubes 4 against the magnetic force of a the strong magnetic field, of 15,000 gauss for example. Particularly in the fine adjustment of the field homogeneity, where thin wires must be inserted in or removed from the non-magnetic tubes 4, the passive shims bend under the influence of the magnetic field, making inserting them into the non-magnetic tubes difficult.
In order to avoid the problems caused by this magnetic force, the magnetic field could be reduced before the adjustment and raised again afterward, but in this case the electromagnets must be energized and de-energized, therefore the time required to adjust the magnetic field increases considerably. Also, in the case of superconducting magnets driven in persistent-current mode, opening and closing the persistent current switch to energize or de-energize the superconducting magnet uses up expensive liquid helium.